straight answer:
xml is good for DESCRIBING informatioin, not for presenting it. Even more than that, you cannot present info with xml only (well you could, but it wont be in very readable format). So if you have some mess you need to be able to describe with few trags - xml is your thing. Look how easy that would be to describe library inventory:

<lib>
<book>
<title>awesome adventures of me</title>
<price>$29.99</price>
<pages>183</pages>
<description>
thats is just an awesome book about my awesome adventures, everybody has to read it!!! its a must!!!
</description>
</book>
</lib>

so just by adding one more <book></book> you can easily increase your info by n # of books. Then simply use some sort of xml parser (whether it is js or php) for outputing info in the way you want. Here come another advantage: you can easily change designed withough even touching the cores of information.
Plus xml is platform independent, so... uh later about that

Of course it doesn't mean you have to use xml everywhere, its just one more tool which is nice to know.